The long-term objective of this proposal is to gain insight into mechanisms of innate immunity to l_ectious agents in humans, with particular emphasis on the role of TolIJike receptors (TLRs) in cutaneous host defense. Significant findings during the current budget period include the following concepts: 1) activation of TLR2 on human dendritic cells triggers DC maturation and induction of IL-12, but not lL*10; 2) induction of direct antimicrobial activity through mammalian TLRs; 3) involvement of receptor interacting protein 2 in innate and adaptive immune responses; 4) IRF3 mediates a TLR3/TLR4- specific arrtMral gene program; 5) TLR2 Ugands as adjuvants for human Thl responses; 6) activation and regulation of TLRs 2 and 1 in human leprosy correlates with clinical outcome; 7) LIR-7 activation antagonizes TLR activatioo of rnonocyte cytokine release. We propose experiments to investigate the role of TLRs in skin and the innate response to microbial infection. First, we hypothesize that different TLRs have distinct roles in host immunity. We will determine the role of specific TLRs in the cutaneous immune response by comparing the distribution of TLRs in skin lesions, analyzing the role of distinct TLRs in Langerhans cell responses to microbial ligands and determining the mechanism by which TLR activation leads to killing of intracellutar bacteria by studying activation of the vitamin D receptor as a model. Second we hypothesize that distinct TLRs or TLR signaling pathways lead to immunostimulatory events. We propose to determine the role of TLRs in phagocytosis and the signaling pathways that lead to phagocytosis. Third, we hypothesize that the expression of TLR on T cells allows these calls of the adaptive immune system to participate in the innate response. We propose to study the ability of TLRs to trigger monocytes to differentiate into DC, and the effect of DC maturation on antigen presentation to T cells. The role of LIRs in countering the TLR induced differentiation of DC will also be investigated. We believe that the insights obtained form the study of TLRs in skin and their response to microbial lipoproteins will provide new knowledge about the innate immune response and suggest new avenues of immunologic intervention in human disease.